This study is an observational cohort to identify the etiologies of acute febrile illness (less than or equal to 2 weeks), which are thought to be endemic or novel in a certain area. Therefore, there are no hypotheses that require sample size calculation to be tested. It is expected that this study will enroll a minimum of 200 patients per site: approximately 100 adults and 100 pediatric cases. The final sample size is therefore estimated to be 1600.